This invention relates to encapsulated integrated circuit and microelectromechanical systems (MEMS) devices. More particularly, this invention relates to the prevention, reduction, elimination or purification of outgassing and trapped gases in such devices.
The ability to maintain a low pressure or vacuum for a prolonged period in a microelectronic package is increasingly being sought in such diverse areas as display technologies, micro-electro-mechanical systems (MEMS) and high density storage devices. For example, computers, displays, and personal digital assistants may all incorporate devices which utilize electrons to traverse a vacuum gap to excite a phosphor in the case of displays, or to modify a media to create bits in the case of storage devices, for example.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. One example of a MEMS device is a microfabricated cantilevered beam, which may be used to detect the presence of a particular material, for example, a biological pathogen, or which may be used in a high-Q gyroscope. By coating the MEMS cantilever with a suitable reagent, the pathogen may bind with the reagent resulting in mass added to the cantilevered beam. The additional mass may be detected by measuring a shift in the characteristic vibration frequency of the cantilevered beam. However, because air is viscous, the cantilevered beam may be required to operate in a vacuum, so that the viscosity of ambient air does not broaden the resonance peak. Accordingly, MEMS devices such as cantilevered beams may also require vacuum packaging, in order to increase the signal-to-noise level of the detector to an acceptable level.
The packaging of the MEMS device may be accomplished by bonding a lid wafer with a device wafer. The MEMS devices, such as the cantilevered beams, are first fabricated on the device wafer. The lid wafer is then prepared by etching trenches or cavities in the lid wafer which will provide clearance for the MEMS device on the device wafer. Before bonding, the lid wafer is aligned with the device wafer, so that the device cavity in the lid wafer is registered above the device on the device wafer, providing clearance for the height of the MEMS device and for its anticipated range of motion.
The lid wafer and device wafer assembly may then be loaded into a wafer bonding chamber, which is then evacuated. The lid wafer is then permanently bonded to the device wafer with a hermetic bond, so that the evacuated environment within the device cavity does not equilibrate with the outside environment by leakage over time.
One of the major problems with vacuum packaging of electronic devices, including MEMS is the continuous outgassing of hydrogen, water vapor, carbon monoxide, and other components found in ambient air, and from the internal components of the electronic or MEMS device. Typically, to minimize the effects of outgassing, one uses gas-absorbing materials commonly referred to as getter materials. Generally a getter material is a metal alloy, for example, an alloy of zirconium (Zr), vanadium (V), and iron (Fe) that is sputter deposited on the surface of the lid wafer. The getter material may then be activated by heating to a predefined temperature, so that the getter desorbs or diffuses the gases already absorbed and is ready to function in the device.
In order to maintain a low pressure, over the lifetime of the vacuum device, a sufficient amount of exposed surface area of the getter material may need to be installed within the package before it is sealed. Accordingly, in order to absorb a larger volume of gas or achieve a lower base pressure, a larger amount of exposed getter material may need to be enclosed in the cavity. This may be a particular problem for the large cavities often disposed above MEMS devices, which may enclose volumes of several cubic millimeters. However, increasing the amount of getter material may increase the size of the device package, and therefore increase its cost.
Alternatively, designs have been proposed which apply getter materials in a cavity which may be located outside the device cavity, but connected to the device cavity by a conduit formed in the device wafer or lid wafer, such that the getter cavity and the device cavity are in gaseous communication with one another. U.S. Pat. No. 6,499,354 describes such a getter cavity in conjunction with a semiconductor microstructure device. However, the need for a getter cavity may increase the die size and therefore also increase the required pitch between devices. As a result, this approach may increase the cost of manufacturing such devices, by reducing the number of devices which can be fit on a wafer substrate.
Accordingly, a design is needed that provides improved gettering without increasing the size of the device package.